Elevating Heaters with No Center Tap?

[Enzo]

Forget for a moment that the 100 ohm resistors are connected to ground or anywhere else. They are in series with each other and across the 6.3v winding. So 200 ohms across 6.3v means about .2W is dissipated. 50 ohms would mean 100 across the 6v and that means about .4W is dissipated.

The whole point of this is to make a virtual center tap - a way to ground the 6v winding. If you use large resistors, it isn't very close to ground.

On the other hand, if you are elevating the heaters, you are applpying a voltage to it, not grounding it, so higher value resistors are fine for that.

[wizard333]

I understand the bit about keeping it close to ground for a virtual center tap now.

How high can one go with the resistors for DC biased heaters? is 1k ok? more? Reason I'm asking is I have 1% values on hand, but not 1% 100ohms or close.

[Enzo]

1%? HArdly critical. If you are talking about a resistor to lift the heaters, it can be most anything

[defaced]

I think he's talking about the two 100 ohm resistors used for the virtual center tap. In which case, I still wouldn't worry. Sure in an ideal world 1% would be best, but as long as they aren't severely mismatched, I think you'll be ok from a hum standpoint.

[wizard333]

Yeah I was talking about the center tap. I figured matching those resistors as close as possible would make it more...............centered! Someone had mentioned above about using 1% resistors and that made sense to me; was just wondering how far one could take that 1% thing.

I also saw where the standard place to pull the bias from is the screen grid supply; is that preferable over the PI or Pre-amp supply for some reason? I can see it being preferable to the OT supply because there is usually 5-7VAC of ripple there, but I'd think the PI or pre-amp supply would be even better since there is very little ripple left there.

[Steve Conner]

1% would be best, but as long as they aren't severely mismatched, I think you'll be ok from a hum standpoint.

My experience is that, in an amp with a hum balance pot for the heaters, the position that gives minimum hum is often quite far from halfway. Maybe 11 o'clock, maybe 2.

So I think in the face of that, the resistor tolerance hardly matters. If you elevate the heaters it should matter even less.

How high can the resistor value be? If it's too high, the heater string can get unbalanced again by capacitive pickup, unbalanced heater-cathode leakage, etc. And signals can feed along it between the tube cathodes. 100 or 220 ohms is probably about right.

[defaced]

My experience is that, in an amp with a hum balance pot for the heaters, the position that gives minimum hum is often quite far from halfway. Maybe 11 o'clock, maybe 2.

That's pretty interesting. Do you have any thoughts as to why? Pot position vs. resistance?

[Steve Conner]

No, it's because the heater-to-cathode leakage and heater emission in the tubes themselves is unbalanced. Spiral filaments help to balance it, but it's still not perfect, and there's no reason to expect that it would be.

For instance, heater emission is caused by cathode material contaminating the heater, and there's no guarantee that it'll splatter onto both ends of the heater equally. Certainly not to within 1%.

[defaced]

Gotcha. There's a non-homogeneous disturbance in the electron cloud.

[Steve Conner]

Yeah, so don't cross the streams.

[defaced]

Ha! Now I'm going to have to find that DVD and watch it this weekend.

[tubeswell]

Ha! Now I'm going to have to find that DVD and watch it this weekend.

Trekkie?

[defaced]

No, I'm generally not into most forms of fiction. I was thinking Ghostbusters where they're on top of the building at the end and Egon says "Don't cross the streams".

[tubeswell]

Ah yeah right - now the '80s all come back to me.